Carbon nanotube carpet on and grown from copper

ABSTRACT

An anode for an electrochemical cell includes a base layer, predominantly of copper, and an interfacial layer from which extends a carpet of carbon nanotubes. The interfacial layer includes an alloy of the copper and a nanotube catalyst from which the nanotubes nucleate and grow. Lithium metal stored within and between the carbon nanotubes forms an active anode layer.

BACKGROUND

An electric battery includes one or more electric cells. Each cellincludes a positive electrode (cathode) and a negative electrode (anode)physically separated by an ion conductor (electrolyte). When a cell isdischarged to power an external circuit, the anode supplies negativecharge carriers (electrons) to the cathode via the external circuit andpositive charge carriers (cations) to the cathode via the internalelectrolyte. During charging, an external power source drives electronsfrom the cathode to the anode and the resultant charge imbalance pullscations from the cathode to the anode via the electrolyte.

Lithium-ion (Li-ion) batteries store charge in the anode as Li cations(aka Li ions). Li-ion batteries are rechargeable and ubiquitous inmobile communications devices and electric vehicles due to their highenergy density, a lack of memory effect, and low self-discharge rate.Lithium-metal batteries store charge in the anode as lithium metal,which is superior to Li ions due to a higher theoretical specificcapacity, lower electrochemical potential, and lower density.Unfortunately, rechargeable lithium-metal batteries have yet to becommercialized, mainly due to the growth of electrically conductivelithium dendrites that can extend from anode to cathode providing adestructive and potentially dangerous internal short. Also troubling,lithium metal produces side reactions with the electrolyte that consumeboth and increase cell impedance. Both dendrites and lithium sidereactions reduce cell life below levels that are commercially viable forimportant markets.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike references refer to similar elements and in which:

FIG. 1 is a SEM image at 80,000× magnification of an active surface ofan electrode 100, a cathode for use in an energy-storage device.

FIG. 2 is a SEM image of the active surface of electrode 100 at 1,000×magnification.

FIG. 3 is an SEM image of electrode 100 in cross section at 4,000×magnification.

FIG. 4 is a flowchart depicting a method 400 of forming electrode 100 tomake e.g. a cathode for an energy-storage device.

FIG. 5 depicts carbon nanotubes 500 at 40,000× magnification.

FIG. 6A depicts a thermogravimetric (TG) plot 600 and differentialscanning calorimetric (DSC) plot 605 of the precursor mixture from step405 of FIG. 4.

FIG. 6B depicts the Raman spectrum of the precursor mixture from step405 of FIG. 4.

FIG. 7A depicts a TG plot 700 and DSC plot 705 of the output from step435 of FIG. 4, the active cathode layer 300 in accordance with theembodiment of FIG. 3.

FIG. 7B is a Raman spectrum of a conductive framework of sulfurizedcarbon showing carbon sulfur (C—S) peaks, sulfur (S) peaks, D, G, and 2Dpeaks.

FIG. 8A plots the cycling performance (charge/discharge) of an electrodein accordance with one embodiment.

FIG. 8B plots the rate performance (charge/discharge) of an electrode (ahalf cell) in accordance with another embodiment.

FIG. 9 depicts an energy-storage device 900, an electrochemical cell,with a cathode 905 and anode 910 separated by an electrolyte 915 andoptional separator (not shown) of e.g. a porous polymer.

FIG. 10 is a scanning electron microscope (SEM) image of anode 910 incross section.

FIG. 11 is a SEM image of anode 910 at a level of magnification thatresolves individual CNTs 1100 and a root area 1105 where the CNTs oflayer 930 connect to interfacial layer 940.

FIG. 12 is a flowchart 1200 illustrating a method of making anode 910 inaccordance with one embodiment.

FIG. 13 is an SEM image of the active portion of a CNT carpet 1300 grownusing a tungsten filament power of 30 W and exposure time of 30 seconds,followed by a CNT growth time of 10 minutes.

FIG. 14 is a Raman spectrum of a carbon nanotube carpet showing RBM, D,G, and 2D modes at about 200 cm⁻¹, 250 cm⁻¹, 480 cm⁻¹, and 2700 cm⁻¹.

FIG. 15 is a plot 1500 of charge/discharge curves showingelectrochemical plating and stripping of Li-metal over and betweencarbon nanotubes at a current density of about 1 mA/cm² or a rate ofabout 0.5 C.

FIG. 16 is a plot 1600 of a cycling experiment showing stable cycling ofa Li-metal plated carbon nanotube carpet at a capacity of about 2mAh/cm², a current density of about 1 mA/cm² or a rate of about 0.5 C.

FIG. 17 is an SEM image of a side-view of an active electrode surface1700 with multiple layers of CNT carpets 1705 on top of one another.

FIG. 18 includes two SEM images 1805 and 1810 of the active portion of aCNT carpet grown using a tungsten filament power of 50 W and exposuretime of 30 seconds, followed by a CNT growth time of 10 minutes.

FIG. 19 depicts a system 1900 for forming anode 910 of FIG. 9 followinga process similar to that detailed above in connection with FIG. 12.

FIG. 20 depicts a stable, high-capacity, rechargeable energy storagecell 2000.

FIG. 21 shows three cross sections of cell 2000 of FIG. 20 in variousstates of charge and discharge.

FIG. 22 shows three cross sections of a cell 2200 like cell 2000 of FIG.20 with like-identified elements being the same or similar.

FIG. 23 depicts a rechargeable energy storage cell 2300 that is similarto storage cell 2000 of FIG. 20 with like-identified elements being thesame or similar.

DETAILED DESCRIPTION

FIG. 1 is a SEM image at 80,000× magnification of an active surface ofan electrode 100, a cathode for use in an energy-storage device. Theactive surface of electrode 100 exchanges lithium ions with anelectrolyte (not shown). Electrode 100 includes a conductive frameworkof tangled nanofibers 105, carbon nanotubes in this example, with lumps110 of amorphous carbon—sulfur distributed within the tanglednanofibers. The amorphous carbon-sulfur lumps 110 are of carbon bondedto sulfur via carbon-sulfur chemical bonds and to nanofibers 105 viachemical bonds. The strength of the chemical bonds secures sulfur atomswithin electrode 100, and thus suppresses the formation of undesirablepolysulfides that would otherwise reduce cell life. Tangled nanofibers105 bind the active materials within electrode 100 while enhancingthermal and electrical conductivities of the active layer.

FIG. 2 is a SEM image of the active surface of electrode 100 at 1,000×magnification. Lumps 110 of various sizes are visible at this level ofmagnification, but the carbon nanotubes of the conductive network aretoo thin to resolve. Carbon nanotubes (tubes of carbon with diametersmeasured in nanometers) are of particularly high tensile strength andexhibit excellent thermal and electrical properties. Nanofibers ofdifferent sizes and types can be used in other embodiments. For example,the tangled nanofibers can include one or a combination of nanotubes,nanoribbons, graphene, carbon fibers, aluminum nanofibers, and nickelnanofibers.

FIG. 3 is an SEM image of electrode 100 in cross section at 4,000×magnification. An active layer 300 of lumps 110 distributed within aconductive network of nanofibers (FIG. 1) is physically and electricallyconnected to an aluminum substrate 305 that serves as a currentcollector when electrode 100 is incorporated into e.g. a capacitor orelectrochemical cell. Active layer 300 is about 50 μm thick, andsubstrate 305 about 20 μm, though this example is not limiting. Activelayer 300 can be relatively dense, advantageously reducing electrolytevolume and thus cell volume. Some embodiments have cell cathode activematerial with a density of 0.4-1.2 g/cm³, a porosity of 20-70%, and apore volume of 0.2-1.8 cm³/g.

Lumps 110 include sulfur that is reacted with and chemically bonded tothe conductive network of nanofibers. Lumps 110 also include amorphouscarbon with both sp2 and sp3 hybridized carbon atoms and are, like thesulfur, chemically bonded to the conductive network of nanofibers. Theratio of sp2 carbon atoms to sp3 carbon atoms is 50-90 at. % sp3 carbonatoms to 10-50 at. %, the sp2 indicative of aromatic rings. The chemicalbonds securing lumps 110 to nanofibers 105 are predominantly covalentbonds. The resultant material is largely a sulfurized amorphous carbonthat is tightly bonded to the conductive framework of tanglednanofibers, though some embodiments include as much as 20 wt % freesulfur, which is to say sulfur that is not chemically bonded to carboneither directly or via an intermediate atom or atoms (e.g., via one ormore sulfur atoms, at least one of which is bonded to carbon).

The chemical stability of the active layer 300 suppresses polysulfideformation and thus allows for relatively high sulfur levels andconcomitant lithium storage. In some embodiments, for example, activelayer 300 includes between 30 and 80 wt % sulfur. Active layer 300 canhave low levels of oxygen, e.g. less than 10 wt %, which reduces therisks associated with thermal runaway. A polymer used in the formationof active layer 300 contributes hydrogen, in one example at aconcentration of between five and twenty atomic percent of the activelayer.

Lumps 110 are largely of amorphous carbon-sulfur with sp2 aromaticcarbon clusters having an average maximum dimension of less than 20 nmdispersed within a matrix of sp3 carbon atoms. Dopants, like nitrogenand oxygen, can be added to improve conductivity and wettability forelectrolyte or solvents. The amorphous carbon-sulfur can include one ora combination of monocyclic or heterocyclic aromatic rings, and theheterocyclic rings can include at least one of oxygen, nitrogen, andsulfur.

FIG. 4 is a flowchart depicting a method 400 of forming electrode 100 tomake e.g. a cathode for an energy-storage device. First, at step 405,nanofibers are mixed with powders of sulfur and a polymer with amolecular weight of between 100,000 Dalton and 1,000,000 Dalton. In thisexample, carbon nanotubes 500 (FIG. 5) are mixed with a powder ofpoly(acrylonitrile-co-vinyl acid), or PAN, with an average molecularweight of 150,000 Dalton, and a powder of sulfur at a mass ratio of 1.5wt %:16.4 wt %:82.1 wt %, respectively. This mixing can be done in apolyethylene container containing zirconia beads using a planetary mixerat 600 rpm for 10 min, then at 1,500 rpm for 10 min, yielding afused/agglomerated powder. Carbon nanotubes 500 are e.g. 500 nm to 10 μmlong and five to one-hundred nanometers in diameter. In someembodiments, the sulfur is admixed in vapor form rather than as apowder.

Next, in step 410, the agglomerated powder from step 405 is crosslinkedand hardened, for example by further mixing at 1,500 rpm for at leastten additional minutes. Crosslinking refers to the formation ofcrosslinks, bonds that interlink polymer chains. Crosslinks can becovalent or ionic bonds. Step 410 heats the mixture to induce thecrosslinking of the precursor, the heat reaching a temperature ofbetween 40° C. and 90° C. The carbon nanotubes act as crosslinking,hardening agents. The crosslinked polymer chains and tangled nanofiberscreate a conductive carbon framework, or scaffold, that maintains thephysical integrity of the crosslinked, hardened mixture duringsubsequent heating. The mixture from step 410 is removed and broken intochunks or pellets. The chunks or pellets from step 410 are ground usinge.g. a mortar and pestle (step 415).

The precursor mix made with tangled carbon nanotubes was much harder andmore abrasion resistant than one without the carbon nanotubes, whichsuggests that the carbon nanotubes play a role in producing hardened andrigid material by providing a rigid framework that supports lumps 110.The fused, hardened properties of the precursor mix from step 415indicate that the transformation was not mere branching of the polymerchains but is also accompanied by crosslinking of the polymer chainsaided by the sulfur and heat, thus restricting mobility of the chainsduring the subsequent high-temperature treatment.

Next, in step 420, the ground, agglomerated powder mixture istransferred to a furnace that is evacuated of air, filled with an inertgas (e.g. argon or nitrogen) and heated at a reaction temperature of450° C. for 6 hours under the inert gas in a quartz tube using asplit-tube furnace. This heat treatment, above the glass-transitiontemperature and below the decomposition temperature of the PAN polymer,pyrolyzes the PAN to chemically bond carbon from the PAN to thenanofibers and the sulfur, thus forming amorphous carbon-sulfurchemically bonded to the nanofibers. The heating additionally drives offconstituent hydrogen and nitrogen, though some hydrogen and nitrogen canremain after the process. Steps 405 through 420 can be carried outabsent some or all of the nanotubes to make sulfurized-carbon granules.Carbon nanomaterials or additional carbon nanomaterials of the same or adifferent type (e.g., ribbons versus tubes of the same or differentlengths) can then be incorporated with the sulfurized-carbon granulesvia mixing and heating. The material is then cooled for e.g. 1 hourswith the aid of a fan (step 425).

Cooled material from step 425 was characterized withthermogravimetric-mass spectroscopy (TG-MS) analysis and a significantmass loss of about 65 wt % was observed upon heating from roomtemperature to 1,000° C., the residual 35 wt % consisting primarily ofcarbon. The lost mass was primarily sulfur, and also included nitrogen,oxygen, and hydrogen that had been bonded to the conductive frameworkwith sulfurized carbon. The sulfur content prior to heating wasdetermined to be about 40 wt % of the cooled material from step 425.

The material from cooling step 425 is mixed with a powdered carbon (e.g.acetylene black), a binder, and an organic solvent or water to form aslurry (step 430). The sulfur in the material from step 425 is stronglybonded to carbon. The resultant chemical stability allows the materialto be combined with inexpensive and environmentally friendly waterwithout producing significant levels of poisonous, corrosive, andflammable hydrogen sulfide. For example, in one experiment using waterto form a slurry, a detector with a detection limit of 0.4 ppm failed todetect hydrogen sulfide. The resistance to hydrogen-sulfide formation isdue to the strong bonding between the sulfur and carbon.

The slurry can contain one or more water-soluble binders, e.g.polyacrylic acid, carboxymethylcellulose, or styrene butadiene rubber.The binder and carbon additive can compose from e.g. 2 to 30 wt % of thesolid mass. The slurry is spread over a conductor (e.g. an aluminumfoil) and dried (step 435) by e.g. freeze drying and/or heating in dryair. The dried cathode layer is compressed e.g. by passing the foilbetween rollers. In an embodiment in which the dried slurry andunderlying foil are together about 100 microns, the compression reducescathode-layer thickness to between 50 and 90 microns, depending on themass loading, with little impact on the foil. Mass loading ofsulfurized-carbon cathodes can be e.g. 2 to 10 mg/cm², with a finalsulfur content of e.g. from 30 to 80 wt %.

“Dry-electrode” embodiments omit steps 430 and 435. Rather than adding aliquid to form a slurry, the material from step 425 can be compressedinto a dry film over a current collector or can be compressed into a dryfilm before application to the current collector. The drying step canthus be omitted. The cathode with the dried, compressed layer from step435 or a dry-electrode process can be incorporated into a lithium-metalcell. During discharge, lithium metal oxidized at the anode releaseslithium ions through the electrolyte to the cathode. An optionallithiation process (step 440) may be used. Lithium ions sourced from,e.g., lithium foil can be electrochemically intercalated into or platedonto a carbon anode layer prior to cell assembly, for example. Othermethods of lithiation are detailed below. Cathodes from method 400 arecompatible with other types of anodes, including those that incorporateporous carbon and silicon to store active metals (e.g., Li, Mg, Al, Na,and K) and their ions.

Returning to FIG. 4, the sulfur content of active layer 300 was variedby tuning the reaction temperature of heating step 420 between 300° C.and 600° C. At temperatures lower than 450° C., the mass loss uponheating during TG-MS analysis was greater than about 65 wt %. Attemperatures above 450° C., the mass loss upon heating during TG-MSanalysis is lower than about 65 wt %. Below 300° C. and above 600° C.,the lithium storage capacity of the electrode made from the material waslower than obtained from materials produced between 300° C. and 600° C.

The size of lumps 110 and the conductivity of active layer 300 can bevaried. In a synthesis similar to the method of FIG. 4, the mixedprecursor material was heated to between 100° C. and 250° C. tocrosslink the precursor material. The crosslinked material was heatedagain, this time to between 300° C. and 500° C. to generate sulfurizedcarbon; and yet again to between 500° C. and 600° C. to promote furthercarbonization and/or graphitization, which increases the size ofgraphitic domains in the sulfurized carbon. Larger graphitic domainsincrease the ratio of sp2 to sp3 carbon, which increases the ratio ofaromatic sp2 to sp3.

The material of step 420 includes graphitic domains or clusteredaromatic carbon rings in the sulfurized carbon. The size of the domainsor clusters can be increased for improved electrical conductivity. Inone embodiment, for example, the domains or clusters were enlarged bysubjecting the material to heat treatments up to a temperature of atleast 600° C. for a period between one microsecond and one minute. Therapid heat treatment was induced by preheating the reactor to atemperature of at least 600° C. and moving the sulfurized carbon from acold zone to the hot zone. These heat treatments also increase the ratioof sp2 to sp3 carbon and reduce hydrogen content. Heat treatment above600° C. for more than an hour leads to a significant decrease in lithiumstorage capacity of the material.

The foregoing method of making an electrode is not limiting. Otherdiscrete or continuous processes can also be used. In one embodiment,for example, the discrete process of FIG. 4 is adapted to a continuousroll-to-roll process in which the active material is formed on one orboth sides of a roll of aluminum foil.

FIG. 6A depicts a thermogravimetric (TG) plot 600 and differentialscanning calorimetric (DSC) plot 605 of the precursor mixture from step405 of FIG. 4. Without crosslinking, the material rapidly loses sulfurabove about 300° C.

FIG. 6B depicts the Raman spectrum of the precursor mixture from step405 of FIG. 4. Raman shifts below about 500 cm⁻¹ indicate the presenceof elemental sulfur.

FIG. 7A depicts a TG plot 700 and DSC plot 705 of the output from step435 of FIG. 4, the active cathode layer 300 in accordance with theembodiment of FIG. 3. With crosslinking and the subsequent heattreatment, the material retains sulfur far beyond the 300° C. of theprecursor from step 450. In one example, 94.4% of the sulfur wasretained up to 450° C. This demonstrates a chemical stability thatprevents active cathode layers of this material from readily decomposinginto polysulfides that escape into the electrolyte.

FIG. 7B is a Raman spectrum of a conductive framework of sulfurizedcarbon showing carbon sulfur (C—S) peaks, sulfur (S) peaks, D, G, and 2Dpeaks. The C-S peaks are indicative of carbon-sulfur chemical bonds, dueto bonding of sulfur to amorphous carbon and the carbon nanotubes of theconductive framework of sulfurized carbon. The S peaks are indicative ofsulfur-sulfur chemical bonds in a sulfur chain attached to the carbon.Thus, some of the sulfur atoms are bonded to only sulfur atoms (S—S) andsome are bonded to both sulfur and carbon atoms (C—S—S). The D, G, and2D modes include contributions from the amorphous carbon and carbonnanotubes in the conductive framework of sulfurized carbon. The D mode,originating from the presence of six-membered rings, is activated by thepresence of defects. The G mode confirms the sp2 carbon structure of thecarbon nanotubes. The 2D mode, an overtone of the D mode, indicates thepresence of six-membered rings and its shape provides structural andelectronic structure about the conductive framework of sulfurizedcarbon. Because the 2D mode is quite noticeable relative to other peaks,it indicates the presence of clustered aromatic rings that provideconductivity in the conductive framework with sulfurized carbon. Thebroadness of the 2D peak confirms the amorphous carbon in the sulfurizedcarbon whereby sp2 carbon atoms are organized as clusters ofsix-membered rings that constitute a short-range order (on the order ofseveral nanometers) before defects such as sp3 carbon, non-carbon atoms,five-membered rings, and/or seven-membered rings, are encountered.

FIG. 8A plots the cycling performance (charge/discharge) of an electrodein accordance with one embodiment. In this example, the electrodematerial includes sulfurized carbon (active material within a sulfurizedframework), carbon black (conductive additive), and polyacrylic acid(PAA binder), at a ratio of 95:5:5, coated from an aqueous (water)slurry on carbon-coated aluminum foil. In one embodiment, thecarbon-coated aluminum comprises an aluminum foil 16 um thick with bothsides coated with a 1 um layer of carbon of an areal density of 0.5g/m². The carbon protects the aluminum from corrosion caused by thefluorinated electrolyte. It also promotes adhesion between the currentcollector and the cathode material. The gravimetric capacity (mAh/g) ofthe electrode is based on the mass of the active material. The mass ofthe electrode material is 5 mg/cm² and the areal capacity at 0.2 C isabout 2.4 mAh/cm².

FIG. 8B plots the rate performance (charge/discharge) of an electrode (ahalf cell) in accordance with another embodiment. The x axis representscharge cycles and the y axis the gravimetric capacity of the sulfurizedcarbon. In this example the active electrode material includes aconductive framework with sulfurized carbon, carbon black (a conductiveadditive), and a polyvinylidene difluoride (PVDF) binder at a ratio of95:5:5. This composition was coated from an N-methylprrolidone (NMP)slurry on carbon-coated aluminum foil that will serve as currentcollector. The mass of the electrode material is 4.5 mg/cm² and theareal capacity at 0.2 C is about 2.2 mAh cm⁻². These data show that thegravimetric capacity of the half cell at 0.2 C recovers after repeatedcharge and discharge cycles at 2 C.

FIG. 9 depicts an energy-storage device 900, an electrochemical cell,with a cathode 905 and anode 910 separated by an electrolyte 915 andoptional separator (not shown) of e.g. a porous polymer. Cathode 905 andanode 910 are each engineered to store relatively large quantities oflithium. Cathode 905 stores lithium in an active layer 920 that includesa conductive framework of sulfurized carbon, as detailed above, over acathode current collector 925 of e.g. aluminum. Anode 910 stores lithiummetal within and between carbon nanotubes (CNTs) of an anode activelayer 930. The CNTs are grown from and secured to a copper currentcollector 935 using an interfacial layer 940 that includes a catalystfor CNT growth.

Electrolyte 915 can be liquid or solid. As a liquid, electrolyte 915 canbe e.g. 4 M lithium bis(fluorosulfonyl)imide with a porous separator ofe.g. 5 μm polyethylene. A solid electrolyte can be used to separateanode from cathode, in which case one or both active layers 920 and 930can incorporate a liquid, paste, or jell electrolyte that facilitatesion flow between the solid electrolyte and the active materials. Theelectrolytes on either side of the solid electrolyte can be the same ordifferent, depending on what best suits the anode and cathode activematerials. Solid, or “solid-state,” electrolytes can be inorganic (e.g.Lithium phosphorous oxynitride (LIPON), Lithium thiophosphate, orLithium nitride) or polymer (e.g. polyethylene oxide).

Lithium-sulfur cathodes can lose sulfur when elemental sulfur reactswith the lithium ions in the electrolyte to form soluble lithiumpolysulfides, which are shuttled between the cathode and anode. In thisdeleterious process, sometimes referred to as the shuttle effect,lithiated polysulfides shuttle sulfur from the active cathode materialthrough the electrolyte to plate onto the anode layer during charging.The shuttle effect both reduces storage capacity and increases internalresistance. Based on information and belief, and without being limitedto theory, an active cathode layer 920 initially lacks or substantiallylacks elemental sulfur. When device 900 is first discharged, thesulfurized carbon is reduced by lithium to form lithium sulfides.Components of electrolyte 915 also reduce within and betweencarbon-sulfur lumps 110 to form an SEI matrix that extends throughcathode active layer 920. The SEI matrix traps the polysulfides but isan ion conductor. During charging, the SEI matrix retains the sulfur andallows lithium ions to escape back through electrolyte 915 to cathodeactive layer 910. The SEI matrix continues to retain the sulfur oversubsequent charge/discharge cycling.

Lithium in anode active layer 930 ionizes to produce lithium ions andelectrons during cell discharge. The electrons power an external load945, passing from anode 910 to cathode 905 via current collectors 925and 935 and the load. Simultaneously, the lithium cations (Li⁺) passfrom anode 910 to cathode 905 via electrolyte 915. Li cations from theelectrolyte reduce sulfur within cathode active layer 920 and formlithium sulfide. Charging reverses this process by stripping lithiumcations and electrons from cathode active layer 920 and returning themto anode active layer 930 where they electroplate the CNTs to form alayer of lithium metal over and between the CNTs.

The capacity of anode 910 is a function of the quantity of lithium metalthat can be stored in active layer 930, while the electrical impedanceis a function of the ease with which charge carriers—Li cations andelectrons—can enter and leave. For storage, the CNT carpet has a massiveareal density, on the order of hundreds or thousands of square metersper gram, that is available for Li plating, yielding lithium storagecapacities (Li mass/CNT mass) of hundreds or thousands of wt %. As forion impedance, the CNTs extend generally in parallel from interfaciallayer 940 so the paths in and out of layer 930 are relatively short andstraight. The electron paths are also of low impedance. CNTs areexcellent conductors, as are the copper and copper alloys of currentcollector 935 and interfacial layer 940. The interfaces between thelayers of anode 910 are low-resistance, ohmic contacts that allow chargeto flow easily in both directions.

Current collector 935 is or includes a base layer predominantly ofcopper. In one embodiment, current collector 935 is an 8 um copper foilthat is 99.9% pure. Interfacial layer 940, formed during the manufactureof anode 910, is of a copper alloy with precipitate particles thatcatalyze and anchor the CNTs of anode active layer 930. Interfaciallayer 940 can include other elements, such as oxygen, that may or maynot catalyze CNT growth. The oxygen may come from native or grownsurface copper oxide. The other elements may include metals, such as Ag,Ni, Cr, Al, Fe, Zn. The other elements may come from unintentionalnative or manufacturing trace impurities or they may be intentionallyintroduced. The other elements may or may not catalyze CNT growth. Inone embodiment, the other elements are less than 20 wt. % of the coppersurface. Based on information and belief, the CNTs have root structuresthat extend out of interfacial layer 940 from the catalyst precipitateparticles and establish strong connections with beneficially low thermaland electrical impedance supported by metallic and covalent bonds.

FIG. 10 is a scanning electron microscope (SEM) image of anode 910 incross section. Interfacial layer 940 between current collector 935 andthe active anode layer 930 is difficult to see. The gray area above theCNT carpet of active layer 930 is empty space but would be filled withelectrolyte in an assembled cell. The term “active” refers to thematerial in contact with the electrolyte that exchanges lithium ions.

FIG. 11 is a SEM image of anode 910 at a level of magnification thatresolves individual CNTs 1100 and a root area 1105 where the CNTs oflayer 930 connect to interfacial layer 940.

FIG. 12 is a flowchart 1200 illustrating a method of making anode 910 inaccordance with one embodiment. First, an iron layer about 5 nm thick isdeposited on a 25 μm copper foil via e.g. e-beam evaporation from aniron target at a pressure of about 5×10⁻⁶ mBar (step 1205). Later in theprocess, as detailed below, the iron from step 1205 is incorporated intothe underlying copper to form precipitate CNT nucleation cites. Ironlayers less than 1 nm thick do not, for this recipe, include enough ironto produce the desired quantity and density of nucleation cites. Thethickness of the iron layer is between three and fifteen nanometers inthis recipe. CNT catalysts other than iron, such as nickel, can be usewith or instead of iron. In this context, a CNT catalyst is any materialthat catalyzes CNT growth.

Next, an aluminum oxide layer about three nanometers thick was depositedover the iron layer via e-beam evaporation from an aluminum oxide targetat a pressure of about 1.6×10⁻⁴ mBar (step 1210). The aluminum oxidelayer protects and constrains the CNT growth so that the CNTs grow inparallel from the interfacial layer. Aluminum oxide layers much belowthree nanometers fail to support adequate CNT growth in this example.

In some embodiments, the catalyst and buffer layers are deposited by oneor more of e-beam evaporation, sputtering, thermal evaporation, atomiclayer deposition, molecular beam epitaxy, electrodeposition,solution-phase deposition, nanoparticle deposition.

In step 1215, the copper foil with catalyst and protective layers isinserted into a load-lock chamber of a three-chamber tubular reactor andthe reactor is prepared for CNT growth. The reactor (not shown) includesa loading chamber, a cold zone, and a hot (growth) zone. The load-lockchamber, with its own pumping and venting systems, is separated from thecold and hot zones of the reactor by a gate valve, while the cold zoneis between the load lock chamber and the hot zone. The hot zone is thepart of the reactor directly under the furnace, while the cold zone isoutside of the furnace. Both hot and cold zones can share the samepumping system. The hot zone is pre-heated to 750° C. under a mass flowrate of 4 sccm acetylene, 200 sccm hydrogen, and 200 sccm hydrogenbubbled through a water cylinder to give a precursor mixture of a CNTsource gas with a total pressure of about 10 Torr. The coated Cu foilsubstrate is introduced to and evacuated in the load-lock chamber usinga vacuum pump down to a pressure below 0.1 Torr.

The gas precursor mixture is activated (step 1220) using a heatedtungsten filament located in the hot zone, which is Joule heated to atemperature of about 2000° C. by a supply of about 30 W of electricalpower from a power supply (step 1220). Then the gate valve is opened andthe substrate transported into the hot zone of the reactor via the coldzone. The tungsten filament produces a characteristic amber glow,indicating activation of an ambient gas mixture of acetylene, hydrogen,and water into various hydrocarbon, hydrogen, oxygen, and hydroxylradicals and neutral fragments, such as atomic hydrogen, acetyleneradical. After about 30 seconds of introducing the substrate to the hotzone, the tungsten filament is powered off. During the 30 seconds ofexposure, the substrate interacts with the thermal energy and activatedchemical species generated by the heated tungsten filament.

In step 1220, during the exposure of the substrate to the activated gasprecursor, the heat from the tungsten filament further increases thetemperature of the substrate above 750° C. for e.g. 30 seconds. Thisthermal treatment diffuses the iron into the copper to create acopper-iron interfacial layer. The iron dissolving in the underlyingcopper eventually saturates the copper and forms precipitate CNTnucleation cites. Thus, the substrate generated after the exposure ofstep 1220 includes copper from the initial foil overlayed with acopper-iron interface, an iron catalyst layer, and a buffer layer ofaluminum oxide at the surface. The interfacial layer is between aboutfive and twenty nanometers, though these layers are not sharply divided;rather, the interfacial concentration of iron is relatively high—e.g.the material predominantly of iron—and decreases into the bulk of thecopper.

Next, in step 1225, the tungsten filament heater is turned off and theCNTs grown using a carbon source gas (e.g. acetylene) and for a timethat depends on the desired properties of the CNT carpet, ten minutesfor a CNT carpet with a height of about 20 μm and an areal mass of about0.1-0.3 mg/cm². The areal mass can be decreased below about 0.1-0.3mg/cm² or increased above 0.1-0.3 mg/cm² by decreasing or increasing,respectively, the catalyst thickness, CNT growth time, total pressure,and/or carbon source concentration or partial pressure. In this example,the source gases are the same for both steps 1220 and 1225. During CNTgrowth, the continuous copper-iron interface atop the predominantlycopper substrate predominantly immobilizes the surface iron at the baseof the carbon nanotube carpet, while some of the aluminum oxide bufferlayer may be present at the interface or lifted with the top of thecarbon nanotube carpet as the CNTs grow from the interfacial layernucleated and catalyzed by the iron precipitates. Some of the iron mayalso be lifted with the grown carbon nanotube carpet. The anodestructure is then removed from the hot zone to the cold zone to rapidlycool e.g. with the aid of a fan (step 1230). In an optional step 1235,lithium metal can be deposited or plated onto the CNT layer.

Some embodiments employ a catalyst layer of iron (e.g. 5 nm) on copperwithout a buffer layer of e.g. aluminum oxide. In one such embodiment,the iron layer is overlayed with iron oxide (e.g. 3 nm), the lattertaking the place of aluminum oxide. In another embodiment, iron (e.g. 5nm) is covered with aluminum oxide (e.g. 3 nm). In yet anotherembodiment, iron and iron-oxide layer (e.g. 5 nm) are covered withaluminum oxide (e.g. 3 nm). Some further embodiments layer iron oxide(e.g. 5 nm) directly over the copper.

In step 1225, CNT length can be increased by increasing process time,pressure, or both. The morphology of the CNT carpet can be controlled byadjusting the electrical power applied to the tungsten filament duringthe alloy formation/nucleation step 1220. The density of the CNT carpetcan be controlled by adjusting the duration of exposure of the substrateto the heated tungsten filament. Other growth parameters being equal,increasing the exposure time from 30 seconds to one minute decreased theCNT carpet density.

In another embodiment of the CNT growth process, the tungsten filamenttreatment described above is omitted when the catalyzed copper foil isinserted into the hot zone of the reactor at 750° C. Even without thetungsten filament treatment, diffusion of the iron into the copperoccurs to create a copper-iron interfacial layer. The iron dissolving inthe underlying copper eventually saturates the copper and formsprecipitate CNT nucleation cites. Thus, the substrate generated afterthe thermal exposure includes copper from the initial foil overlayedwith a copper-iron interface, an iron catalyst layer, and a buffer layerof aluminum oxide at the surface. The interfacial layer is between aboutfive and twenty nanometers, though these layers are not sharply divided;rather, the interfacial concentration of iron is relatively high—e.g.the material predominantly of iron—and decreases into the bulk of thecopper.

In other embodiments of the CNT growth process, the growth temperatureis between 550° C. and 700° C. In another embodiment of the process, thecopper is catalyzed on both sides for dual-sided growth of CNT. Thecatalyzed copper can be suspended in the reactor, without sitting on aplatform and only held at the edges, thus facilitating simultaneousgrowth of CNT on both sides of the copper. A tension or force can beapplied at the edges of the copper to move it through the reactor.

In another embodiment, the catalyzed copper is vertically oriented inthe reactor, facilitating simultaneous growth of CNT on both sides ofthe copper.

The foregoing method of making an electrode is not limiting. Otherdiscrete or continuous processes can also be used. In one embodiment,for example, the discrete process of FIG. 12 is adapted to aroll-to-roll process in which the active material is formed on one orboth sides of rolled metal (e.g. copper) foil.

FIG. 13 is an SEM image of the active portion of a CNT carpet 1300 grownusing a tungsten filament power of 30 W and exposure time of 30 seconds,followed by a CNT growth time of 10 minutes.

FIG. 14 is a Raman spectrum of a carbon nanotube carpet showing RBM, D,G, and 2D modes at about 200 cm⁻¹, 250 cm⁻¹, 480 cm⁻¹, and 2700 cm⁻¹.The RBM (radial breathing mode) is indicative of single-walled carbonnanotubes. D mode, originating from the presence of six-membered rings,is activated by the presence of defects. G mode confirms the sp2 carbonstructure of the carbon nanotubes. The 2D mode, an overtone of the Dmode, indicates the presence of six-membered rings and well-developedelectronic structure and conductivity of the carbon nanotubes.

FIG. 15 is a plot 1500 of charge/discharge curves showingelectrochemical plating and stripping of Li-metal over and betweencarbon nanotubes at a current density of about 1 mA/cm² or a rate ofabout 0.5 C. The electrode area is about 2 cm². The test subject, a CNTcarpet grown on copper in the manner described above, was placed insidea standard 2032 coin cell with a Li metal chip as a counter andreference electrode, a 5 μm polyethylene separator, and a 4 M lithiumbis(fluorosulfonyl)imide electrolyte.

FIG. 16 is a plot 1600 of a cycling experiment showing stable cycling ofa Li-metal plated carbon nanotube carpet at a capacity of about 2mAh/cm², a current density of about 1 mA/cm² or a rate of about 0.5 C.The electrode area is about 2 cm².

FIG. 17 is an SEM image of a side-view of an active electrode surface1700 with multiple layers of CNT carpets 1705 on top of one another.Surface 1700 was grown with a tungsten filament power of 50 W over anexposure time of 30 seconds followed by a CNT growth time of 10 minutes.

FIG. 18 includes two SEM images 1805 and 1810 of the active portion of aCNT carpet grown using a tungsten filament power of 50 W and exposuretime of 30 seconds, followed by a CNT growth time of 10 minutes. Images1805 and 1810 are of the same material but taken at different levels ofmagnification.

FIG. 19 depicts a system 1900 for forming anode 910 of FIG. 9 followinga process similar to that detailed above in connection with FIG. 12. AnE-beam evaporator 1905 running at 10 kW is used to successively depositfirst iron then alumina, each at a vapor pressure of less than 10⁻⁴Torr, on a copper foil. The resulting substrate is moved to areactor/furnace 1910, a tube that is 2.54 cm in diameter, to undergo achemical-vapor-deposition (CVD) process at 750° C.

A heated pipe 1915, heated to 100° C., preheats reactants forreactor/furnace 1910. In this example, the reactants are acetylene at 4sccm at 0.5 Torr, hydrogen at 200 sccm and 4.4 Torr, and a combinationof hydrogen and water at 200 sccm and 2.9 Torr. Reactor/furnace 1910includes a loading chamber, a cold zone, and a hot (growth) zone. Theload-lock chamber, with its own pumping and venting systems, isseparated from the cold and hot zones of the reactor by a gate valve,while the cold zone is between the load lock chamber and the hot zone.The hot zone is the part of the reactor directly under the furnace,while the cold zone is outside of the furnace, and both hot and coldzones share the same pumping system.

The hot zone is pre-heated to 750° C. under a mass flow rate of 4 sccmacetylene, 200 sccm hydrogen, and 200 sccm hydrogen bubbled through awater cylinder to give a total pressure of about eight to ten Torr. Acopper foil coated with catalyst and buffer layers is introduced intoreactor/furnace 1910 and evacuated in the load lock chamber using avacuum pump 1920 down to a pressure below 0.1 Torr. A tungsten filamentin the hot zone is Joule heated to a temperature of about 2,000° C. by asupply of about 30 W. The tungsten filament activates the ambient gasmixture of acetylene, hydrogen, and water into various hydrocarbon,hydrogen, oxygen, and hydroxyl radicals and neutral fragments, such asatomic hydrogen, acetylene radical. The foil substrate is moved to thehot zone while the filament is powered.

The tungsten filament is powered off after about 30 seconds ofintroducing the substrate to the hot zone. During the 30 seconds ofexposure, the substrate interacts with the thermal energy and activatedchemical species generated by the heated tungsten filament. The heatfrom the tungsten filament further increases the temperature of thesubstrate above 750° C. This thermal treatment diffuses the iron intothe copper to create a copper-iron interfacial layer. The irondissolving in the underlying copper eventually saturates the copper andforms precipitate CNT nucleation cites. Next, in the same zone but withthe tungsten filament powered down, a CNT carpet is grown from theinterfacial layer. The resultant anode 910, unspent reactants, andreaction byproducts are conveyed to a cooling tube 1925. Vacuum pump1920 removes the unspent reactants and reaction byproducts (collectivelythe “effluent”) and returns them to heated pipe 1915 for recycling. Thecooled anode 910 is then removed from the system.

Anodes of the type detailed above can be used with sulfur-basedcathodes. Conventional lithium-sulfur batteries are notable for theirhigh specific energy but suffer relatively short cycle lives that havelimited adoption. Cells in accordance with some embodiments combine ahigh-capacity anode with a highly stable sulfurized-carbon cathode.

Many variations and modifications of the structures, methods, andmaterials disclosed herein are possible and are within the scope of theinvention. For example, the CNT material employed as the active portionof an anode in an electrochemical cell could also be used in otherhigh-surface-area applications, such as VANTABLACK coatings or otherelectrodes for electrochemical cells or capacitors.

FIG. 20 depicts a stable, high-capacity, rechargeable energy storagecell 2000 similar to cell 900 of FIG. 9, with like-identified elementscan be the same or similar. A metal anode 2005 (a first electrode) ismatched with a sulfur-based cathode 2010 (a second electrode). Theelectrodes are separated by an electrolyte 2015 with a 2017 of, e.g., aporous polymer. Anode 2005 includes a current collector 2020 of, e.g.,copper physically and electrically connected to an anode layer 2025 ofporous carbon saturated with an organic liquid electrolyte 2015. In oneembodiment, anode layer 2025 comprises a carbon-nanotube (CNT) carpet.Suitable methods of forming a cathode layer of CNTs are detailed above.Cathode 2010 includes a current collector 2030 of, e.g., aluminumphysically and electrically connected to a porous cathode layer 2035that can be saturated with electrolyte 2015. An alkali-metal layer 2037(e.g. of lithium) in electrolyte 2015 and between electrodes 2005 and2010, on either or both sides of the separator 2017, is in contact withthe external surface of at least one of porous layers 2025 and 2035 butis initially separated from the internal surfaces (in the pores orinterstices) of both porous layers. The lithium metal of layer 2037 isionized and moved between anode 2005 and cathode 2010 when cell 2000 ischarged and discharged.

Cathode layer 2035 is a nanoporous carbon-sulfur composite, a mixture ofporous carbon and sulfur. The porous carbon collectively forms a matrixthat improves thermal and electrical conductivity, traps harmfulpolysulfides that would otherwise migrate away from the cathode 2010 andaccommodates expansion and contraction that accompanies the addition anddepletion of lithium. Detailed treatments of cathode materials suitablefor cathode layer 2035 are detailed above.

The structure of the carbon or graphene scaffolding facilitateslithium-ion transport while trapping polysulfides. This structure isfashioned without admixed lithium metal that might otherwise interferewith the formation of that structure. The absence of lithium at thecathode is compatible with lithium-metal anodes. The absence of lithiumis not compatible with anode 2005, however, a carpet of carbon nanotubesthat is initially formed devoid of lithium.

Lithium layer 2037 can be a continuous or perforated lithium foil, themetal of which becomes the active material in cell 2000. The mass oflayer 2037 is selected such that both cathode layer 2035 and anode layer2025 have the capacity to store the entire amount, between twenty andforty microns thick in one non-porous embodiment. Because essentiallyall the lithium is employed, cell 2000 exhibits improved specificcapacity and energy density relative to conventional lithium-ion anodesin which the amount of lithium is generally lower than the amount ofcarbon that stores the lithium ions.

FIG. 21 shows three cross sections of cell 2000 of FIG. 20 in variousstates of charge and discharge. Beginning with the uppermost example,labeled 2000, the active material within cells 2000 exists primarily inlithium layer 2037, with some lithium ions dissolved in the electrolyte(the separator is not shown). While within a discrete layer for ease ofillustration, electrolyte 2015 can occupy the empty spaces within porousanode and cathode layers 2025 and 2035. Both sides of lithium layer 2037may thus be in contact with electrolyte 2015. The cathode side oflithium layer 2037 is in physical and electrical contact with cathodelayer 2035 and is physically and electrically separated from layer 2025.In other examples, lithium layer 2037 may be in contact with eitheranode layer 2025 or cathode layer 2035 with only one lithium surfaceexposed to the electrolyte. During electrolyte injection, someelectrolyte may find its way into whichever porous layer is in contactwith lithium layer 2037. Embodiments in which layer 2037 is e.g.granular or otherwise porous facilitate wetting of the underlying layer.

The middle example of cell 2000 is labeled 2000C, the “C” for charging.A power supply 2100 draws electrons from cathode 2010 and consequentlithium ions from anode layer 2037 as the lithium metal is oxidized. Ina process called “electrostripping,” layer 2037 is depleted as thematerial migrates as lithium ion through the separator and electrolyte2015 to coat the interior surfaces of anode layer 2025, which is labeled2025Li to note this modification. Though not shown, lithium layer 2037essentially disappears when the constituent metal is depleted. Thoughnot shown, a passivating SEI forms on CNT surfaces of layer 2025L fromdecomposition products of the electrolyte. The SEI passes lithium ions,blocks electrons, and prevents further electrolyte decomposition. In thedepicted embodiment, anode layer 2025Li supports essentially all thelithium from layer 2037 as a coating of metallic lithium within andbetween carbon nanotubes. Cell 2000C is thus fully charged. Experimenthas shown that pre-wetting the porous layer adjacent lithium layer 2037is not necessary, as the electrolyte is drawn into porous surfaceseither during electrolyte injection or lithium-layer depletion.

The lowermost example of cell 2000 is labeled 2000D, the “D” fordischarging. A load 2105, represented as a resistor, allows electronsfrom anode 2005 to migrate toward cathode 2010 as lithium ions fromanode layer 2025Li concomitantly migrate toward cathode 2010 to take upresidence within the porous cathode layer to form lithium sulfides. Thelithiated cathode layer is labeled 2035Li to note this modification. Theratio of lithium in anode layer 2025Li to the total lithium in anode andcathode layers 2025 and 2035 essentially determines the state of charge,the higher the ratio the higher the state of charge. Layer 2037 does notreform during subsequent charge and discharge cycles. A coating oflithium metal reversibly forms within the porous anode layer 2025 (e.g.,between CNTs).

In one embodiment, anodes and cathodes are separately fabricated intoelectrode sheets, which are then cut into desired shapes to form anodes2005 and cathodes 2010. A sheet of separator material and lithium foilare likewise cut to desired shapes to form a separator and lithium layer2037. Cell 2000 is assembled from these materials and filled withelectrolyte. Lithiation then proceeds byelectrostripping/electrodeposition to charge cells 2000 as illustratedby cell 2000C. In another embodiment, lithium layer 2037 is initiallydeposited, on either the anode or the cathode, by e.g. physical vapordeposition. Thermal evaporation of lithium, for example, can be used toproduce a lithium layer with good adhesion to the target surface.

FIG. 22 shows three cross sections of a cell 2200 like cell 2000 of FIG.20 with like-identified elements being the same or similar. The maindifference is that cell 2200 includes a lithium layer 2205 (e.g. lithiumfoil) on the anode side of electrolyte 2015. As in the example of FIG.21, cell 2200 is shown in various states of charge and discharge.Beginning with the uppermost example, labeled 2200, the active materialwithin cell 2200 exists primarily in lithium layer 2205, with somelithium ions dissolved in the electrolyte on either side of layer 2205.The anode side of lithium layer 2205 is in physical and electricalcontact with anode layer 2025 and is physically and electricallyseparated from cathode layer 2035. In some embodiments the lithium oflayer 2205 can be divided in two, e.g. one layer on either side of theseparator. For example, electrolyte injected into a cell can carrylithium particles that form layers of the metal on both sides of theseparator.

The middle example shows cell 2200D discharging though a load 2105.Electrons and lithium ions migrate from anode layer 2025 to populate theinterstices of porous cathode layer 2035Li. The lowermost example showscells 2200C charging responsive to a power supply 2100 that drawselectrons from cathode 2010 and consequent lithium ions from cathodelayer 2035Li. Cathode layer 2035Li is depleted as the material migratesto the interior surfaces of anode layer 2025Li, thus forming a coatingof metallic lithium within and between carbon nanotubes. Cell 2000C isthus fully charged. Layer 2205 does not reform during subsequentdischarge cycles. Cell 2200 can be assembled in the manner detailedabove in connection with FIG. 21, the difference being placement oflithium foil 2205 and starting with a discharge (cell 2200D) rather thana charge.

FIG. 23 depicts a rechargeable energy storage cell 2300 that is similarto storage cell 2000 of FIG. 20 with like-identified elements being thesame or similar. This embodiment is lithiated with a layer 2305sandwiched between cathode active layer 2035 and cathode currentcollector 2030. When cell 2300 is thus assembled, porous cathode layer2035 allows electrolyte 2015 to create an ion path from lithium layer2305 to CNT layer 2025. When cell 2300 is first charged, the lithiummetal of layer 2305 is ionized and moved to CNT layer 2025. Cathodelayer 2035 absorbs the lithium metal during subsequent discharges tothat lithium layer is or is largely absent in normal use.

Lithium layer 2305 is shown on only one side of cathode currentcollector 2030 but can be on both sides and can be applied to either orboth sides as a discrete film or films. In a continuous process, forexample, a perforated 20 um lithium foil is applied to both sides of thealuminum current collector by roller and pressure. In other embodiments,the lithium layer or layers can be formed on the current collector. Inone embodiment, for example, lithium layer 2305 is electrodeposited to athickness of 20 μm in an electrolyte comprising a lithium salt dissolvedin an organic solvent, e.g. 4 M lithium bis(fluorosulfonyl)imide. In oneembodiment, the deposition is carried out at a current density of about0.4 mA cm⁻² for about 10 hours, producing deposited lithium passivatedby solid electrolyte interphase comprising decomposition products of theelectrolyte.

The foregoing discussion focuses on electrochemical cells that employlithium ions as charge carriers. Other alkali metals (e.g. sodium andpotassium) can also be used. Moreover, while the lithium layers arecontinuous films in the foregoing examples, metal layers can beintroduced as e.g. perforated sheets, screens, or loose or agglomeratedparticles, wires, or rods that assemble into a layer during deviceassembly. A slurry of metal particles and electrolyte can be used inlieu of or with the electrolyte.

Variations of these embodiments will be obvious to those of ordinaryskill in the art. Therefore, the spirit and scope of the appended claimsshould not be limited to the foregoing description. Only those claimsspecifically reciting “means for” or “step for” should be construed inthe manner required under the sixth paragraph of 35 U.S.C. § 112.

What is claimed is:
 1. An electrode comprising: a base layerpredominantly of copper at a first concentration; an interfacial layeron the base layer, the interfacial layer including copper at a secondconcentration and a carbon-nanotube catalyst; and carbon nanotubesextending from the interfacial layer.
 2. The electrode of claim 1,wherein the interfacial layer comprises an alloy of the copper at thesecond concentration and iron.
 3. The electrode of claim 2, wherein thecarbon-nanotube catalyst includes precipitates of the iron.
 4. Theelectrode of claim 3, wherein the carbon nanotubes extend from theprecipitates of the iron.
 5. The electrode of claim 1, wherein thesecond concentration is lower than the first concentration
 6. Theelectrode of claim 1, the interfacial layer including a catalyst layeropposite the base layer.
 7. The electrode of claim 6, wherein thecatalyst layer is predominantly of a metal other than copper.
 8. Theelectrode of claim 7, wherein the metal other than copper comprisesiron.
 9. The electrode of claim 1, wherein the interfacial layer is of athickness between three and twenty nanometers.
 10. The electrode ofclaim 1, the carbon nanotubes further comprising a second metal.
 11. Theelectrode of claim 10, wherein the second metal comprises aluminum. 12.The electrode of claim 10, wherein the second metal is of aconcentration in the carbon nanotubes that varies in proportion to adistance from the interfacial layer.
 14. The electrode of claim 1,wherein the carbon-nanotube catalyst is of a metal with an interfacialconcentration in the interfacial layer and a lower concentration in thebase layer.
 15. The electrode of claim 1, wherein the interfacial layeris in ohmic contact with the base layer.
 16. The electrode of claim 4,wherein the interfacial layer is in ohmic contact with the carbonnanotubes.
 17. The electrode of claim 1, wherein most of the carbonnanotubes are bonded to the interfacial layer by at least one metallicbond.
 18. The electrode of claim 1, further comprising a substratesupporting the base layer opposite the interfacial layer.
 19. Theelectrode of claim 1, wherein the copper includes a copper-oxide layer.20. The electrode of claim 1, wherein the copper includes a coppersurface comprising elements other than the copper constituting less than20 wt. % of the copper surface.
 21. A method of forming nanotubes on anelectrode comprising a base layer predominantly of copper, a catalystlayer of a nanotube catalyst on the base layer, and a protective layerover the catalyst layer, the method comprising: exposing the protectivelayer to a nanotube source gas; heating the electrode, the heatingproducing an interfacial layer on the base layer, the interfacial layercomprising an alloy of the copper and the nanotube catalyst; and growingthe nanotubes between the interfacial layer and the protective layer.22. The method of claim 21, the nanotubes lifting the protective layerfrom the interfacial layer in consequence of the growing.
 23. The methodof claim 21, further comprising obtaining the electrode by forming thecatalyst layer over the base layer and forming the protective layer overthe catalyst layer.
 24. The method of claim 21, wherein the catalystlayer comprises iron.
 25. The method of claim 21, wherein the protectivelayer comprises an oxide of aluminum.
 26. The method of claim 21,further comprising absorbing the protective layer into the nanotubesduring the growing.
 27. The method of claim 21, wherein the nanotubesare carbon nanotubes.